1. Field of the Invention
The present invention relates to optical fiber communications, and particularly to optical add-drop multiplexers comprising a bandpass filter and installed in an optical dense wavelength-division multiplexer (DWDM) communications network for dropping an optical signal having a particular wavelength from an optical wavelength-division multiplexed signal and simultaneously complementing the optical wavelength-division multiplexed signal by adding an optical signal having a wavelength that is identical to the particular wavelength of the dropped optical signal.
2. Description of Prior Art
An optical DWDM communications network based on an optical wavelength-division multiplexed signal is widely used in large-scale communications networks. In order to set up a complete optical communications network based on optical DWDM technology, optical add-drop multiplexers should be adopted to drop and add optical signals having particular wavelengths and thereby exchange information. A variety of optical add-drop multiplexers have been developed.
FIG. 1 shows an optical add-drop multiplexer 70, as disclosed in U.S. Pat. No. 6,101,012. The optical add-drop multiplexer 70 comprises an input optical fiber L2 and an output optical fiber L3 which are connected with a network. An optical multiplexed signal B from the input optical fiber L2 is transmitted into an input end 711 of a DWDM 71, and demultiplexed into optical signals B1, B2, . . . , BL respectively comprising different wavelengths. Then, the optical signals B1, B2, . . . , BL are transmitted to corresponding output ends 712, 713, . . . , 714 of the DWDM 71. B1 is transmitted into one input end of a 2×2 optical exchange device 72, via an optical fiber L13. The other input end of the optical exchange device 72 is connected with an optical fiber L5 comprising an added optical signal A1. Two output ends of the optical exchange device 72 are respectively connected with an optical fiber L15 which is used to transmit optical signals, and an optical fiber L6 which comprises a dropped optical signal D1. B1 is changed into C1 by the optical exchange device 72. In this process, an optical signal D1 having a particular wavelength is dropped from C1, and an optical signal A1 having the same wavelength as D1 is added to C1. C1 is transmitted into an input end 732 of a DWDM 73. In the same way, B2, . . . , BL are respectively changed into C2, . . . , CL, and transmitted into corresponding input ends 733, . . . , 734. Then, C1, C2, . . . , CL are multiplexed into C, and C is transmitted to an output optical fiber L3 via an output end 731. The optical add-drop multiplexer 70 further comprises a converter 74, a controller 75, a modulator 76, an attenuator 77, and a converter 78.
The above-described optical add-drop multiplexer 70 has several drawbacks. For example, interference between signals is easily produced, because an input optical multiplexed signal needs to be demultiplexed. Then optical signals having particular wavelengths need to be added and dropped by the optical exchange devices, and multiplexed with the demultiplexed optical signals.
In addition, the DWDMs 71 and 73 comprise many optical devices. Therefore the DWDMs 71 and 73 occupy too much space, are unduly heavy, and are costly. Furthermore, a plurality of long optical fibers L5, L6, L13, L15, L17, . . . , L23 are placed in the optical add-drop multiplexer 70. Thus the optical fibers L5, L6, L13, L15, L17, . . . , L23 are easily pulled apart or connected in error, resulting in high insertion loss in each joint.
FIG. 2 shows another conventional optical add-drop multiplexer 90, as disclosed in U.S. Pat. No. 5,822,095. The optical add-drop multiplexer 90 comprises a first circulator 93, a bandpass filter 94 and a second circulator 98. The bandpass filter 94 has a central wavelength λ1 that passes an optical signal having a wavelength λ1, but reflects optical signals having other wavelengths.
In use, an input optical multiplexed signal from an optical fiber 91 is transmitted from a node 931 to a node 932 of the first circulator 93, then transmitted to the bandpass filter 94 via an optical fiber 95. An optical signal having a wavelength identical to the central wavelength λ1 of the bandpass filter 94 is passed through the bandpass filter 94 and transmitted to a node 981 of the second circulator 98 via an optical fiber 99. Then the optical signal having wavelength λ1 is dropped to an optical fiber 96 via a node 982. Other optical signals having other wavelengths are returned to the first circulator 93 by the bandpass filter 94, and output to an optical fiber 92 via a node 933. An added optical signal having wavelength λ1 from an optical fiber 97 is transmitted from a node 983 to a node 981 of the second circular 98. That added optical signal is then transmitted to the first circular 93 via the bandpass filter 94, and output to the optical fiber 92 via the node 933.
The above described optical add-drop multiplexer only deals with the optical signals having the wavelength to be added or dropped. Thus, interference between signals can be reduced. However, in practice, a plurality of optical add-drop multiplexers 90 needs to be connected in series to add or drop multicenter optical signals. An optical add-drop multiplexer module comprising a plurality of optical add-drop multiplexers 90 occupies considerable space. In addition, the circulators 93, 98 each comprise at least two optical devices. The overall result is high costs. Furthermore, a multitude of optical fibers 91, 92, 95, 96, 97 and 99 needs to be adopted during the assembling process. The optical fibers 91, 92, 95, 96, 97 and 99 are difficult to identify, and are easily pulled apart or connected in error. Moreover, the resulting multitude of joints increases insertion loss.